1. Field of the Invention
The present invention generally relates to an apparatus and method for low-noise amplification in a wireless communication system. More particularly, the present invention relates to a low-noise amplification apparatus and method for selectively according to a radio channel status in a wireless communication system.
2. Description of the Related Art
In wireless communication systems, a receiver typically uses a Low Noise Amplifier (LNA) for amplifying the weak power of a received signal, while suppressing noise in the received signal. Therefore, the requirements for the LNA include a low noise factor, a high gain, and safety.
The receiver typically controls the gain of the LNA according to the status of a radio channel by changing a voltage applied to the LNA. For example, the receiver controls the LNA according to a radio channel environment as illustrated in FIG. 1.
FIG. 1 is a flowchart illustrating a conventional procedure for operating an LNA in a wireless communication system. The following description is made on the assumption that a first threshold (threshold 1) is larger than a second threshold (threshold 2) and threshold 2 is larger than a minimum sensitivity.
Referring to FIG. 1, the receiver measures the Received Signal Strength Indication (RSSI) of a radio channel in step 101.
In step 103, the receiver compares the RSSI with threshold 1 to ascertain whether the RSSI is greater than Threshold 1 in order to determine an LNA operation scheme according to a radio channel environment. Threshold 1 can be 80 dBm, for example.
If the RSSI is larger than threshold 1, in step 105 the receiver sets the LNA to bypass mode, considering that the radio channel is sufficiently strong by bearing greater than the Threshold 1. That is, the receiver controls a received signal to bypass the LNA.
On the contrary, if the RSSI is equal to or less than Threshold 1, at step 107 the receiver compares the RSSI with threshold 2 to ascertain whether the RSSI is larger than Threshold 2, which is essentially a determination as to whether Threshold 1 is greater than or equal to the RSSI, and whether the RSSI is greater than Threshold 2.
If the RSSI is larger than threshold 2, at step 109 the receiver sets the gain of the LNA to a medium gain, considering that the radio channel is at an intermediate strength.
On the other hand, if the RSSI is equal to or less than threshold 2 in step 107, the receiver compares the RSSI with the minimum sensitivity in step 111 to determine if the RSSI is larger than the minimum sensitivity. The minimum sensitivity is a minimum requirement of the radio channel status for the receiver to decode a signal.
If the RSSI is larger than the minimum sensitivity, at step 113 the receiver sets the gain of the LNA to a high gain, considering that the radio channel is relatively weak (lower than both Threshold 1 and Threshold 2).
If the RSSI is equal to or less than the minimum sensitivity, at step 115 the receiver determines that it cannot receive the signal because of a bad radio channel status.
Then, the receiver ends the algorithm.
As described above, the receiver amplifies a received weak signal and suppressing its noise, by using a different LNA operation scheme depending on the RSSI of the radio channel. However, when the receiver is affected by interference from a neighbor channel, the LNA circuit becomes saturated due to an interference-caused third-order harmonic frequency.
Accordingly, the LNA cancels the third-order harmonic from the received signal by use of a main amplifier and a sub-amplifier as illustrated in FIG. 2.
FIG. 2 is a circuit diagram of a conventional LNA in the wireless communication system.
Referring to FIG. 2, an LNA 200 includes a main amplifier 210 and a sub-amplifier 220. The main-amplifier 210 amplifies a signal received through a base (A) and outputs the amplified signal to a collector (B). The signal output from the collector B has the inverted phase of the signal input to the base A.
The sub-amplifier 220 amplifies a third-order harmonic component more strongly than a signal component in the same signal as a signal input to the main amplifier 210. The sub-amplifier 220 uses two transistors 221 and 223 to avoid inversion of the phase of the signal.
Thus, as shown in the conventional circuit in FIG. 2, the LNA 200 combines a signal with an inverted phase amplified by the main amplifier 210 with a signal with the phase kept intact amplified by the sub-amplifier 220, thereby canceling the third-order harmonic component.
A gain controller 230 controls the gain of the LNA 200 according to a radio channel environment. If the radio channel is weak, the gain controller 230 sets the resistance of a resistor R0 to a high value so that the LNA 200 gets the highest gain. If the radio channel is intermediate in strength, the gain controller 230 sets the resistance to a low value so that the LNA 200 gets a medium gain.
As described above, the LNA shown in FIG. 2 can suppress noise by canceling a third-order harmonic component by use of the main amplifier and the sub-amplifier. Yet, the signal component is also attenuated when the third-order harmonic component is canceled through combination of the signals output from the main amplifier and the sub-amplifier. As a consequence, the Signal-to-Noise Ratio (SNR) of the LNA decreases.